Long Term Evolution (LTE) is the name given to the latest standardization work by the 3rd generation partnership project (3GPP). LTE adopts a flatten network architecture. For example, the radio access network (RAN) includes a radio base station (RBS), but not a radio network controller (RNC). An LTE RBS is also referred to as an evolved node-B (eNB), which is a key component of the LTE RAN.
LTE support two modes of operation: (1) Frequency Division Duplex (FDD) and Time Division Duplex (TDD), for the configuration on paired and unpaired frequency bands. For these two modes, the basic differences are located in the physical layer (e.g., physical layer frame structure). In 3GPP, FDD uses frame structure type-1 (FS1); TDD works in frame structure type-2 (FS2). However, the layers above the physical layer (e.g., media access control (MAC) layer and higher layers) are the same for both FDD and TDD. That is, most (e.g., over 90%) of the signaling is similar between the two modes of operation.
For these two types of frame structures, the basic difference is that FS2 has a special sub-frame 1 ms out of 5 ms or 10 ms in periodic. In principle, this special sub-frame is inherited from TD-SCDMA frame structure. Three special time slots (DwPTS, GP, UpPTS) locate in the special sub-frame of 1 ms, used for cell search and radio access and so on.
Even if, FDD will be the main mode of operation in most systems, the TDD mode of operation will also be highly regarded by some key network operators. As mentioned above, the eNB is the kernel of the LTE RAN. From the standard compatibility and network entity integration point of view, both the FDD and TDD modes of operation are expected to be implemented in one node. But there are some hard problems that need to be solved with respect to including both FDD necessary components and TDD necessary components into one eNB cabinet.